Drilling operations, such as oil or natural gas drilling, are often conducted using electrical equipment, such as sensors, data storage and transmission devices, located within a drilling collar. The electrical equipment is usually inserted within the drilling collar used to transmit torque from the surface to the drill bit. Electrical power for the equipment is often supplied by one or more batteries.
The use of batteries to power electrical equipment located within a drill hole can present disadvantages. For example, batteries require periodic replacement. The need to replace batteries can cause interruptions in drilling operations. The down-time associated with such interruptions can result in substantial losses in revenue. Moreover, the cost of replacement batteries over time can be substantial.
The amount of power available from batteries can be relatively limited. In particular, it can be difficult to obtain the amount of power required for certain applications from a battery small enough to fit within the limited confines of a drilling collar. Also, batteries are not particularly well suited for exposure to the relatively high temperatures that can occur within a drill hole during drilling operations.
Alternators (or direct-current generators) can be used as an alternative power source to batteries. For example, an alternator can be equipped with a turbine that drives the alternator. The turbine can be driven by the passage of drilling mud therethrough. (Drilling mud (mud slurry) is commonly pumped through the drilling collar from the surface during drilling operations. The drilling mud helps to cool the drill bit, clear the drill bit of drilling debris, and carry cuttings to the surface.)
The use of an alternator (or generator) to power electrical equipment located in a drill hole can present disadvantages. For example, the wiring used to transmit signals to and from the electrical equipment can be difficult to route over the alternator. Hence, the alternator is usually positioned above or below the electrical equipment it powers. This arrangement can interfere with (or prevent) the use of certain types of electrical equipment that need to be located below the other equipment in the drilling collar.
Turbine-driven alternators can be susceptible to contamination by the drilling mud. In particular, the static pressure of the drilling mud increases with the depth of the drill hole, and can be extreme near the bottom of a relatively deep drill hole. Hence, an inflow of drilling mud into components such as bearings can occur if adequate precautions are not taken to seal the components. Moreover, the magnets of the alternator, if not isolated from the drilling mud and casing scale, can attract and retain the metallic debris, such as drill-bit shavings, that is usually present in drilling mud. This debris can interfere with or damage the magnets, and can result in jamming.
The overall form factor of the turbine-driven alternator can make it difficult to fit a turbine-driven alternator within the relatively narrow confines of a drilling collar in some applications. These difficulties can be exacerbated by the need to make the components of the turbine-driven alternator strong enough to resist the substantial mechanical stresses associated with drilling operations.